1. Field of the Invention
The present invention relates to a photodiode (semiconductor light receiving element) used in a receiving module of an optical communication system. More particularly the present invention relates to the photodiode for having a signal light inputted in parallel with a substrate and performing a photoelectric conversion on it.
2. Description of Related Art
As a photodiode used in a receiving module of an optical communication system, there are waveguide photodiode and an evanescently coupled photodiode.
Since a conventional waveguide photodiode has a structure in which an absorption layer of an input face is directly irradiated with an input light, a photoelectric current tends to concentrate near the input face. For example, in case that a high-intensity light such as an output light from an Erbium-doped fiber preamplifier is inputted, there has been a problem that the input face of a waveguide photodiode is liable to be destroyed.
On the other hand, an evanescently coupled photodiode has a structure in which a signal light is inputted to a guide layer being a transparent semiconductor layer the signal light and is wave-guided from the input portion to a PD region (photoelectric converter portion) formed at a distance of several ten microns or more from the input portion. In a PD region (photoelectric converter portion) of an evanescently coupled photodiode a light exuding in the direction of layer thickness from a guide layer (an evanescent wave) is photoelectrically converted in an absorption layer. Therefore, since an input light is indirectly coupled with an absorption layer in the PD region, concentration of a photoelectric current in the vicinity of an input face is more relieved in comparison with a waveguide photodiode and the input face is made to be hard to destroy even in case of having a high-intensity light inputted to it.
Examples of such an evanescently coupled photodiode having the resistance to a high-intensity optical input has been reported, in xe2x80x9c60th Convention of Japanese Institute of Applied Physics in Fall 1999, Proceedings Part 3, pp.985, Lecture No.1p-ZC-8xe2x80x9d (Reference paper 1) and in xe2x80x9cELECTRONICS letters 25th May 2000, Vol.36, No.11, pp.1-2xe2x80x9d (Reference paper 2).
FIG. 1A and FIG. 1B show the basic structure of an evanescently coupled photodiode reported in reference paper 1, and as shown in FIG. 1B, FIG. 1A is a plan view of a prior art example and FIG. 1B is a sectional view of it taken along line A-Axe2x80x2. As shown in FIG. 1B, this photodiode has a layered structure comprising an n+-InP clad layer 102, an n+-InAlGaAs guide layer (of 1.3 xcexcm in wavelength and 1 xcexcm in layer thickness) 103, an i-InGaAs absorption layer 104 (of 0.5 xcexcm in layer thickness), a p+-InP clad layer 105 and a p+-InGaAs contact layer 106 which are formed on a semi-insulating InP substrate 101. In this evanescently coupled photodiode, an input waveguide region 108 is formed over a length of 20 xcexcm from the input face and a PD region (photoelectric converter portion) 107 is formed at the back of this input waveguide region. This input waveguide region is an optical waveguide having the n+-InAlGaAs guide layer 103 as a core layer. The PD region 107 is formed at the back of the input waveguide region. Reference paper 1 has reported that an evanescently coupled photodiode is not deteriorated even in a state of having a high-intensity light of 10 mA in photoelectric current inputted into it.
An evanescently coupled photodiode has a structure in which a PD region 7 is formed distantly from an end face having a signal light inputted to it and there is not a semiconductor layer in front of (between an input face and) an i-InGaAs absorption layer 4. Due to this, differently from a conventional waveguide photodiode, light is little inputted through a side face of the i-InGaAs absorption layer 4 near the input face. Accordingly, the density of a photoelectric current in the vicinity of this side face is very low. In case of comparing this embodiment with a conventional evanescently coupled photodiode, in such a way the conventional evanescently coupled photodiode is increased more in resistance to a high-intensity optical input in comparison with a conventional waveguide photodiode, but its resistance to the high-intensity optical input has not necessarily been sufficient in practical use. Even in the evanescently coupled photodiode, heat locally generated in an area where the density of a photoelectric current is high at the input side of the PD region may sometimes exceed the resistance of the photodiode.
An object of the present invention is to provide a photodiode for having a signal light inputted in parallel with a substrate and performing a photoelectric conversion on it, said photodiode being more improved in resistance to a high-intensity optical input.
In order to attain the above object, according to an aspect of the present invention, there is provided an evanescently coupled photodiode comprising a semiconductor substrate, a guide layer which is made of a semiconductor material whose band gap wavelength is shorter than the wavelength of an input light, said semiconductor material being larger in refractive index than said semiconductor substrate, and which has a signal light inputted to it, an absorption layer which is made of a semiconductor material whose band gap wavelength is equal to or longer than the wavelength of an input signal light and which is formed distantly from the input face of the guide layer and in which a signal light is evanescent-wave-coupled, and a PD region for performing a photoelectric conversion on a signal light inputted in parallel with the semiconductor substrate in an area including at least the guide layer and the absorption layer, wherein the mesa width of the PD region is made wider near the end portion at the side to which a signal light is inputted and more narrow near the end portion at the opposite side to it. And in order to attain the above object, according to another aspect of the present invention, there is provided a waveguide photodiode comprising a semiconductor substrate, an absorption layer made of a semiconductor material whose band gap wavelength is equal to or longer than the wavelength of an input signal light, and a PD region for performing a photoelectric conversion on a signal light inputted in parallel with said semiconductor substrate in an area including at least said absorption layer, wherein the mesa width of said PD region is made wider near the end portion at the side to which a signal light is inputted and more narrow near the end portion at the opposite side to it.